1. Field of the Invention
The present invention generally relates to implantable medical devices such as implantable cardiac pacemakers and implantable cardioverter/defibrillators, and in particular to a capacitive feedthrough for filtering off external interference signals, e.g. defibrillation signals, in order to protect the electronic circuits and telemetry circuits of such an implanted device.
2. Description of the Prior Art
Implantable medical devices, including cardiac rhythm management devices such as pacemakers and implantable cardioverter/defibrillators, typically have the capability to communicate data with an external programmer via a radiofrequency telemetry link. A clinician may use such an external programmer to program the operating parameters of an implanted medical device. For example, the pacing and other operating characteristics of a pacemaker are typically modified after implantation in this manner. Modern implantable devices also include the capability for bidirectional communication so that information can be transmitted to the programmer from the implanted device. Among the data which may typically be telemetered from an implantable device are various operating parameters and physiological data. The implantable device generates and receives the radio signals by means of an antenna. Today, antennas capable of far-field communications are of increasing interest in implantable medical devices, which allows communication over much greater distances than inductively coupled antennas.
The technology of cardiac pacemakers has developed in sophistication and functionality over the years. In general, cardiac pacemakers are designed to control the heart by correcting or compensating for various heart abnormalities which can be encountered in human patients. For example, cardiac pacemakers may provide therapeutic stimulation to the heart by delivering therapeutic pulses such as pacing, cardioversion or defibrillation pulses. However, with this increasing sophistication has come a concomitant increase in sensitivity of the implantable devices to misoperation due to external influences such as defibrillation, electrocautery, and the like. Such interference or voltage pulses may be received by the antenna, for example, at locations where galvanic exposure against tissue occurs and may be conducted further into the electronic circuits and RF telemetry circuits of the medical device. This may cause the medical device to falsely identify the interference as being of cardiac origin and give rise to, for example, an erroneous output rate, or in worst case, it may change the state of or destroy components of the circuits, which, in turn, may severely damage the functions of the medical device.
Conventional implantable devices, such as cardiac pacemakers and implantable cardioverter/defibrillators, are therefore generally provided with protection circuits or filter circuits adapted to protect the electronic circuits and RF telemetry circuits of the medical device against undesired voltage pulses, i.e. to filter off external voltage transients or pulses. Conventionally, a capacitance is arranged within the device and connected in series with the antenna and the electronic circuits and RF telemetry circuits as protection circuit or filter circuit adapted to filter off external interference signals such as defibrillation signals. Thus, RF signals are transferred via the antenna lead through the galvanically conductive feedthrough passing through the serial capacitor while external interference signals are filtered off.
However, this conventional solution is impaired with a number of drawbacks. For example, the filter circuit, i.e. the capacitor, requires extra space, which may be a problem in implantable devices, such as cardiac pacemakers, where internal space is limited. Moreover, since the external interference signals are transferred into the hermetically sealed device, they may give rise to interference leakage within the device.
Hence, there is a need for an improved circuit that in an effective way can protect the internal circuits of an implanted medical device against undesired voltage pulses caused by exposure to e.g. defibrillation and/or electrocautery.